Electronic devices typically require a connected (wired) power source to operate, for example, battery power or a wired connection to a direct current (“DC”) or alternating current (“AC”) power source. Similarly, rechargeable battery-powered electronic devices are typically charged using a wired power-supply that connects the electronic device to a DC or AC power source. The limitation of these devices is the need to directly connect the device to a power source using wires.
Wireless power transfer (WPT) systems typically use time-varying magnetic fields and the principle of magnetic induction or magnetic resonant induction to transfer power wirelessly. In accordance with Faraday's Law, a time-varying current applied to a transmitter coil produces a magnetic field that will induce a voltage in a receiver coil that is in close proximity to the transmitter coil. The induced voltage in the receiver coil is typically rectified and filtered to produce a substantially direct current (DC) voltage that can provide power to an electronic device or a rechargeable battery. Such wireless power transfer systems may use magnetic induction or magnetic resonant induction techniques, both of which emit magnetic flux in the “near-field.” Such near-field techniques are capable of transferring power only when the transmitter coil and the receiver coil are within a short distance from one another, typically on the order of a few centimeters or less.
The Wireless Power Consortium (WPC) was established in 2008 to develop the Qi inductive power standard for charging and powering electronic devices. Powermat is another well-known standard for WPT developed by the Power Matters Alliance (PMA). The Qi and Powermat near-field standards operate in the frequency band of 100-400 kHz. The problem with near-field WPT technology is that typically only 5 Watts of power can be transferred over the short distance of 2 to 5 millimeters between a power source and an electronic device, though there are ongoing efforts to increase the power. For example, some concurrently developing standards achieve this by operating at much higher frequencies, such as 6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methods instead of magnetic induction, they are based on the same underlying physics of magnetic induction. There also have been some market consolidation efforts to unite into larger organizations, such as the AirFuel Alliance consisting of PMA and the Rezence standard from the Alliance For Wireless Power (A4WP), but the technical aspects have remained largely unchanged.
FIG. 1 is a diagram of a prior art embodiment of a single coil structure for wireless power transfer. A transmitter 100 includes a DC voltage source 110, a half-bridge inverter circuit 112, a resonant capacitor 114, and a coil 116. Coil 116 is typically a flat spiral coil with a predetermined number of turns. Half-bridge inverter circuit 112 is controlled by a control circuit (not shown) to provide an alternating current to capacitor 114 and coil 116. The current is typically in the range of 100 KHz to 400 kHz. The capacitance value of capacitor 114 and the inductance value of coil 116 determine a resonant frequency for transmitter 100. The alternating current passing through coil 116 generates magnetic flux that can induce a current in a receiver coil (not shown).
One drawback of single coil wireless power transmitters is that the area of the transmitter coil is limited by the magnetic field necessary to induce a sufficiently large current in a receiver coil. This limitation results from the fact that the magnetic flux produced by a coil is inversely proportional to its area. A small coil in the power transmitter makes its alignment with the receiver coil in the device to be charged more critical. But merely enlarging the area of a spiral coil will cause the magnetic flux generated by the coil to be weaker, particularly in the middle of the coil. One option for overcoming this limitation is to use multiple coils instead of a single coil. A multiple of identical coils can cover a greater area while each coil can generate a sufficient magnetic flux for effective power transfer.
FIG. 2 is a diagram of a prior art embodiment of a multiple coil structure for wireless power transfer. A transmitter 200 includes a DC voltage source 210, a half-bridge inverter circuit 212, a capacitor 214, and multiple coils 220, 222, and 224 connected in series. Coils 220, 222, and 224 collectively provide a larger area that emits magnetic flux. But the multiple coil arrangement in FIG. 2 has a drawback. Connections 240 and 242 between coils 220, 222, and 224 and connection 244 between coil 224 and ground create an unintended loop 230 of parasitic inductance. Loop 230 can become an inadvertent emitter of unwanted electromagnetic interference (EMI). Thus there is a need for a technique for increasing the transmitting area of a wireless power transmitter that does not introduce unwanted EMI.